Naïve CD4⁺ helper T (TH) cells, upon activation by antigen‐presenting cells (APC), differentiate into different types of effector cells that are characterized by their distinct cytokine production profiles and immune regulatory functions. In addition to TH1 and TH2 cells, a third subset of effector TH cells has recently been described and termed TH17. Since their identification, TH17 cells have emerged as crucial players in infectious, inflammatory, and autoimmune diseases, and cancer. In this review, we summarize the latest discoveries on the cytokine‐mediated regulation and transcriptional programming of TH17 cells and their roles in different immune responses and diseases.